Valence bond order in a honeycomb antiferromagnet coupled to quantum phonons

We use exact quantum Monte Carlo simulations to demonstrate that the N\'eel ground state of an antiferromagnetic SU(2) spin-$\frac{1}{2}$ Heisenberg model on the honeycomb lattice can be destroyed by a coupling to quantum phonons. We find a clear first-order transition to a valence bond solid state with Kekul\'e order instead of a deconfined quantum critical point. However, quantum lattice fluctuations can drive the transition towards weakly first order, revealing a tunability of the transition by the retardation of the interaction. In contrast to the one-dimensional case, our phase diagram in the adiabatic regime is qualitatively different from the frustrated ${J}₁\text{\ensuremath{-}}{J}₂$ model. Our results suggest that a coupling to bond phonons can induce Kekul\'e order in Dirac systems.